Picking a Wireless Standard

Is There Currently a Leading Standard for Connecting Distributed I/O and Sensors That's a Safe Pick for the Future?

By Control Design Staff

Our printing machines have widely distributed I/O and sensors, and we could save money by connecting some of these points via a wireless network. We hesitate to proceed because we're not sure where the current wireless standards are headed, and we don't want to implement a supposed standard that will fall out of favor in a few years. Is there currently a leading standard that would be a safer pick for the future?

—From October '12 Control Design

Answers

Standards Aren't the Thing
Wireless can indeed lower costs and even improve reliability in some cases (for example, eliminating possible cable failures due to harsh environments or maintenance issues encountered with slip rings/festooning on moving machines). But the savings are only achieved if the appropriate wireless technology is chosen and the wireless network is designed correctly. Reliability and compatibility are often more important than complying with a standard.

The first step is to review the current control system architecture — is it possible to adapt parts to wireless? The distributed I/O might be using some type of fieldbus, and a few (such as Ethernet) are adaptable to wireless. Sensor interfaces also vary, from simple 4–20 mA loops, to more sophisticated fieldbuses. It might be necessary to redesign the automation network to better accommodate wireless.

From a wireless I/O standard perspective, there are two dominant standards today: WirelessHART (IEC 62591) and ISA100.11a. Both have multi-vendor support and promise to have good market longevity. However, both standards are focused primarily on wireless field devices (instruments) for the process control markets. These networks are field-proven to be highly reliable and robust, but they are based on RF frequency-hopping techniques (IEEE 802.15.4) that adapt well in noisy environments but may have too much jitter (delay) and too slow scan rates for applications in discrete processes.

I suspect that your printing machine distributed I/O and sensors require faster scan rates than what WirelessHART or ISA100.11a support. If the system is PLC-based, easy PLC integration may be more important than following a wireless I/O standard. If so, a Wi-Fi-based solution (such as 802.11n) might be the best choice. The 802.11n standard offers very high-speed data connections for Ethernet, and most PLC vendors support Ethernet (EtherNet/IP, Profinet and Modbus TCP/IP being the most popular). Using wireless Ethernet (Wi-Fi) then provides a high-speed, low-latency connection to Ethernet-based distributed I/O blocks and sensors. These types of wireless solutions have been used in manufacturing and by machine builders for many years.

The best approach is to determine which technology is the best fit for your application. Standards come and go, but fortunately industrial communication vendors offer products with long lifecycles. Evaluate the company as well as the technology/products. How many years have they been in business? How much industrial wireless experience do they have? How knowledgeable about your application are their support engineers? Wireless can indeed lower costs, but choosing the best technology for your application is the key to success.

Jim Ralston,
strategic wireless manager,
ProSoft Technology

Explore 802.11n
Connecting I/O to sensors and actuators through a wireless network can save money, but you need to examine these I/O points and determine whether any of these points are deterministic and/or mission-critical. For example, a wastewater treatment facility could use a wireless sensor to indicate that a tank has reached capacity. Whether the PLC then communicates wirelessly, instructing the valve to turn off the flow in 0.01 s or 1 s is not relevant. A printing press, bottling plant or motion-oriented application might not have that degree of freedom.

If you feel comfortable with the wireless response time you can get in your printing machine application, you will want to explore Ethernet 802.11n. It is the leading standard for wireless Ethernet, and it offers improvement over susceptibility to interference from a broad range of wireless devices and industrial infrastructure. The 802.11n was designed with the goal of reaching 100+ Mbps, but is now projected to be reliable at 200+ Mbps. In an industrial setting, this gives the user quite a bit of room to grow.

The 802.11n standard offers the robustness, throughput, security and quality of service (QoS) capabilities that plant managers have come to expect from wired Ethernet networks. Part of this robustness is an added stream of data — 802.11n uses four beams instead of three. It also allows wireless transmissions to be shaped to arrive in phase when they get to the receiver. All other signals that arrive already out of phase are then cancelled out, cutting down interference and further improving reliability.

{pb}Ethernet is fast becoming the leading connectivity standard for industrial applications and has a track record of more than 20 years of offering "future-proof" connectivity solutions. The 802.11n wireless standard is compatible with the earlier Ethernet and wireless Ethernet standards, and is also operable with legacy standards, and it will allow equipment to be upgraded in the future without any infrastructural changes.

Other options are wireless fieldbuses such as WirelessHART and other non-proprietary communication standards that have been in existence for some time.

Sven Burkard,
industrial networking solution manager;
Raymond DiVirgilio,
Belden field solutions manager,
Lumberg Automation

Consider Your Requirements
Going with a standard is great thinking, but going wireless is not the only consideration. Data requirements (speeds, how often it's needed, transmission distance) and the environment should also enter into the decision process.

The four most common wireless standards for sensor connectivity are Wi-Fi, Bluetooth, ZigBee and cellular. Each of these standards serves a wide variety of applications and possesses advantages and disadvantages, outlined in the table. I believe all these standards are here to stay; I don't think we will see a "Beta vs. VHS" type debate in our lifetime.

So how do we pick one? For your printing press application, let's make the following assumptions:

• Data will come in small amounts, but will be needed frequently and quickly, requiring speed.

• Sensors will monitor roller speeds, ink levels, power, paper level, vibration and emergency failover.

• None of these sensors will require high resolution, so the amount of data will be small, but high-speed printers require high-speed monitoring, therefore data speed is required.

My safe pick here would be Wi-Fi. Wi-Fi is a well-accepted, heavily deployed, tested technology, and there will be an explosion (in the billions) of deployed machine-to-machine (M2M) devices in the next few years.

About the other technologies:

Cellular is a huge part of the M2M world. Although cellular would be considered too expensive to monitor the individual sensors we spoke of, it would be a perfect fit for aggregating all the sensor data to one location. If a company had many geographically dispersed printing presses, sensors could collect data needed to schedule maintenance and inventory items such as paper, ink, etc.

ZigBee is a good option if data isn't needed quickly, but device interoperability can be tricky. Devices labeled "ZigBee" might mean that the device is compatible in a ZigBee environment, but not necessarily interoperable with other ZigBee devices. Said differently: The device won't interfere with other ZigBee devices, but won't communicate with them either.

Bluetooth is useful for connecting devices with very fast, streaming data, such as a headset to a cellphone. I'm seeing Bluetooth's increased use in sensor systems, but nowhere near the levels of Wi-Fi or cellular.

Standards have matured from the thinking of yesterday. With the billions of devices coming online in the near future, it's clear to me why the industry requires sanity. Developers and manufacturers worldwide are demanding better access and a voice to the market through a standards body. I am a delegate to the new oneM2M alliance, launched in July 2012 by seven information and communications technology standards development organizations (SDOs). The goal of this alliance is to develop specifications to ensure the connectivity and communication between M2M devices worldwide.

Bill Conley,
cellular and proprietary RF product manager,
B&B Electronics

Eliminate the Need for Standards
There never will be a single, leading wireless standard; the technology is evolving too rapidly, and design tradeoffs inherently result in different systems.

For example, consider that the iPhone supports GSM, EDGE, LTE, Wi-Fi a/b/g/n 2.4 and 5 GHz, Bluetooth — over a dozen different wireless protocols in all. When wireless is used in industrial settings, the situation necessarily becomes more focused on performance and needs than interoperability. In fact, you could use Wi-Fi today, but the range is too short and the link is not reliable enough for most industrial applications. Instead of waiting for an industrial wireless standard, choose a wireless system based on your application needs.

Choosing the correct vendor for wireless is similar to choosing a PLC vendor; you will need to depend on this vendor, so choose carefully. Good technical support representatives that can help you reliably address system or equipment failures can greatly increase your wireless system's lifespan.

Sustainable, long-lasting wireless systems will also consider power management, durability and scalability. Focus on the characteristics of your wireless applications:

• What is the required response time?

• Do the wireless points need to be battery powered?

• What range is required?

• How many sensing points or locations are there?

• How much data traffic will there be?

Of the above considerations, the need for battery-powered wireless sensors is the most important. If the end points need to be battery-powered, they will also need to be very efficient, putting serious constraints on the types of sensors that can be used. It also slows response time and reduces range. All of the characteristics trade off with one another, so be flexible in your requirements. For example, determine the longest possible response time you can live with, the shortest possible range, and the smallest amount of sensing points.

The reliability and features required in industrial applications has long been defined by PLC systems. When choosing a wireless system, look for PLC-level functionality, reliability and determinism. This will ensure your monitoring and control needs are being met now and into the future. In proprietary systems where the wireless network is developed from the ground up, the sensor and nodes are designed to work well together. This compatibility tends to allow users more flexibility when customizing the sensing system to their specific needs.

By engaging with a well-known, established vendor and creating wireless systems that address your specific needs, you eliminate the need for standards and are able to focus on creating long-lasting efficient systems that stand the test of time.

Bob Gardner,
senior product manager,
Banner Engineering

Bluetooth Workaround
Don't let apprehension prevent you from benefiting from the convenience and economy of wireless networking. It significantly streamlines project costs (engineering/install time, labor, materials) and supports networking options that were previously impossible or cost-prohibitive.

A Bluetooth Ethernet gateway could be an ideal workaround. Both protocols are deeply entrenched and widely supported, providing a cost-effective entry to wireless networking. Replacing communication lines between a PLC and distributed I/O, a gateway wirelessly transmits Ethernet protocols, such as Profinet, Modbus TCP or EtherNet/IP, via Bluetooth 2.0. In addition to linking with distributed I/O and sensors, you get peace of mind — the protocols are well-established and growing in application scope.

To support printing machinery, such as the above-mentioned application, well-developed gateways can relay data/protocols up to 400 m/1,300 ft (Class 1) within a building. This should be more than adequate for wirelessly linking large-scale printing machinery to distributed I/O and sensors.

To further address concerns users may have, advanced gateways implement technology such as adaptive frequency hopping and low emission mode for coexistence with other wireless systems, such as WLAN. They also have features such as an internal, circularly polarized antenna that bolster radio connection reliability in metal-intensive environments.

Charlie Norz,
product manager – Wago-I/O-system,
Wago

Wi-Fi Everywhere
There is currently one, universal, agreed-upon wireless standard in existence today that applies to the type of network the machine builder needs to use: Wi-Fi. Wi-Fi is an IEEE specification (802.11) that has been in use since the early 2000s. Various versions of Wi-Fi exist, beginning with 802.11b to 802.11n today, and in most cases these versions are backward-compatible. And, though another wireless technology could supersede 802.11 a/b/g/n in the future, usually new standards are backward-compatible.

Wi-Fi is ubiquitous and used almost everywhere — from commercial to residential to industrial settings. Security is continually addressed by the largest networking hardware suppliers and software vendors in the world. Wi-Fi is inexpensive, well-understood, and it's easy to obtain tools to troubleshoot and build Wi-Fi-based solutions.

Anything based on 802.15.4 (mesh-networkable, silicon-based wireless technology) falls short of being standardized, particularly regarding the stack, or transport layer. Technologies include ZigBee, 6LowPAN, proprietary networks, and various incarnations of ISA100.

If you want to implement wireless today, stick with a standard technology, and future-proof your machine for at least five to 10 years. Then your choices really narrow down to wireless products based on Wi-Fi (IEEE 802.11).

Ben Orchard,
systems engineer,
Opto 22

Wireless Investment Security
Over the past couple years, wireless LAN (WLAN) has worked its way to become the top pick for most industrial applications. This is especially because of its easy integration with existing Ethernet nodes and the widespread use of the WLAN standard over devices like laptops and smartphones. This allows the use of one infrastructure for not only the I/O and sensor connection back to the PLC, but also quick and easy access from mobile devices for diagnostic purposes.

If you choose WLAN for your application, you are definitely choosing a strong and leading platform with support over many years to come. WLAN was first defined in the IEEE 802.11 standard 1997, and the first real standards — 802.11b and 802.11a — were released in 1997. The 802.11b standard allows up to 11 Mbps and uses the ISM frequencies in 2.4 GHz; and 802.11a operates in the 5 GHz ISM frequencies and offers up to 54 Mbps. In 2003, 802.11g was ratified. This standard also operates in 2.4 GHz, but increased maximum gross data rates up to 54 Mbps.

The newest standard being used is 802.11n, which was officially released in 2009. This standard can be used on 2.4 and 5 GHz, and increased the data rate even further, theoretically up to 600 Mbps. And most importantly, the 802.11n standard is downwards-compatible with 802.11a/b/g, which is important as many new applications are being built today using 802.11a/b/g.

The flexibility and widespread use of WLAN, combined with the proven acceptance and adaptability of the 802.11 standard, continue to provide machine builders and end users the investment security they need to expand their use of wireless solutions today and in the future.

Tim Pitterling,
product manager,
industrial Ethernet infrastructure,
Siemens Industry

The Right Network
When looking at today's currently available wireless network technologies, there isn't a single one-size-fits-all wireless standard that will meet all application needs. It's important to look at the specific context or problem you are trying to solve, and to evaluate constraints such as needed bandwidth, distances between nodes, number of nodes, existing spectrum use/interferences and deployment environments.

For example, Profinet I/O typically runs over a wired Ethernet network (copper or fiber), but can also be run over an 802.11 wireless LAN (WLAN). These WLANs provide a lot of bandwidth, but they might not be appropriate for all environments. This is why the PNO has also created a Wireless Sensor Area Network (WSAN) standard to complement and address areas where WLAN is not a fit. Though WSANs support a very large number of nodes, they support a lot less bandwidth (i.e., less data). Since both 802.11 and 802.15.1 use the same portion of the ISM radio spectrum, they must be configured such that there is enough bandwidth available to allow all nodes to communicate and not interfere with each other.

Moving to the appropriate wireless network can save costs and provide greater flexibility.   Though we do not think there will be a single leading standard, there will be several key ones from which you can choose according to your specific operating context and what you'd like to accomplish through using a wireless network.

David Elliott,
senior engineer,
GE Intelligent Platforms

Stick With Wi-Fi
Proprietary technologies have their place, such as in low-power process instrumentation meshes. However, for most industrial applications, standard Wi-Fi technology offers the greatest interoperability with PCs, tablets and other radios; the widest range of vendors; and more rapid technology development.

We would recommend that control strategies be built on IEEE 802.11n standards using the 5 GHz band. These standards leverage a multiple-input, multiple-output (MIMO) antenna structure for more robust Wi-Fi communications. Packet aggregation for this protocol enhances the wireless efficiency in many automation applications. We believe that these standards also provide for the best forward and backward compatibility of infrastructure solutions. The 5 GHz band is dedicated to RLAN and is relatively uncongested, offering more available bandwidth.

Paul Brooks,
business development manager,
networks portfolio,
Rockwell Automation